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Milky Way’s central bar stirs in stars like a candy floss mixer

Stretched thin

NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

By Joshua Sokol

THE Milky Way’s centre hosts a propeller stirring up chaos in its wake. That’s one way to explain a fraying ribbon of stars called the Ophiuchus stream. Looking at similar stellar streams with this in mind may help us map our galaxy from the inside out.

That’s no easy feat, because Earth is embedded in the Milky Way galaxy’s disc. From here, material blocking our view means we can’t easily map the bar of stars at the spiral’s centre, or the halo of dark matter surrounding the galaxy. But we can see structures orbiting above and below the disc, like dwarf galaxies and tight beehives of stars called globular clusters.

We also see streams of stars, many of which stretch across tens of thousands of light-years: the strewn wreckage of Milky Way satellites that have been torn apart. When a globular cluster falls through the galactic disc, the kick of energy it receives can make it burst like a piÑata, scattering stars in a thin line along the path it used to follow. Over time, the stream stretches and lengthens.

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But one stream, in the direction of the constellation Ophiuchus, looks peculiar. It’s unusually short, which implies either that the cluster it came from burst all at once about 400 million years ago – or that it has been shedding stars here and there for billions of years.

Adrian Price-Whelan of Columbia University in New York favours the latter explanation. His team argues that the Ophiuchus stream has been fraying at its edges due to the chaotic gravitational influences from the Milky Way, and claims the galaxy’s central bar is to blame (arxiv.org/abs/1601.06790).

Simulations suggest that the core of the Milky Way hosts a dense bar of stars that spins around inside the galactic disc like a propeller or a pinwheel. In Price-Whelan’s model, gravity from that rotating central bar has been unravelling threads of stars from the globular cluster that birthed the Ophiuchus stream for billions of years, then churning them into the Milky Way like strands of candy floss.

“All we’re seeing now is that last clump of stars slowly spreading out into a stream,” Price-Whelan says. “In another 500 million years or so that will disappear – that will be dispersed into chaos.”

The same mixing process could allow the Milky Way to hide past crimes. Long, thin stellar streams mark where dwarf galaxies or globular clusters were cannibalised, but the stars left behind from other episodes may be hard to find if they have been stirred into the galaxy. This could help explain why we see fewer satellites than expected: the streams marking their demise may have been erased.

“The galaxy hosts a dense, central bar of stars that acts like a propeller, stirring chaos in its wake“

Studying the short Ophiuchus stream may also help us map the bar itself, says Ana Bonaca of Yale University, who was not part of the research. “It is really the location of the stream that makes it so interesting,” says Bonaca. Identifying more stars at the thin edges of the stream will help us better understand how the bar stirs them in.

Price-Whelan’s colleagues are already looking for more of those fringe stars. The European Space Agency’s Gaia spacecraft, which maps stars across the galaxy, should help, allowing us to use tattered streams to understand the shape of the inside of the Milky Way and the halo of dark matter around it.

This article appeared in print under the headline “Milky Way’s bar stirs up stars like candy floss”